1. Field of the Invention
The present invention relates to a sound barrier wall for use along roadways, railroads, factories, etc. to reduce a noise coming from such source.
2. Description of the Prior Art
For controlling noises from roadways, railways, factories, etc., sound or noise barrier walls have so far been proposed for preventing direct propagation of a noise from such source. Among a wide variety of noise control structures having been proposed for such purposes, the noise barrier wall is relatively inexpensive and effective for deadening sounds from various sources. For a higher effect of noise control, the noise barrier wall should be correspondingly higher. However, the increased height of the barrier wall will add to the costs of noise control (including the cost of fabrication, installation and maintenance, etc.), and also raise many problems such as the shading of sunlight, blocking of viewing, claustrophobia, poor ventilation, radio jamming, air turbulence, etc.
For a higher effect of noise control than attainable only by a straight wall structure of noise barrier, there are currently available a top bent type noise barrier wall of which the top end is bent towards a noise source, a curved type one of which the top end is curved towards a noise source, and the like. However, these types of noise barrier walls raise more serious problems than mentioned above.
Recently, the traffic volume has increased more and more and the traffic speed has become increasingly high, so that the environmental pollution by noises has become a more serious social problem. However, there have not yet been proposed any other effective solutions to the serious problem of traffic noise. Sound barrier walls of the above-mentioned straight type, top-bent type and curved type are still used which are designed to have an increased height of 5 m, 7 m or 10 m only for the purpose of noise deadening, while the above-mentioned problems incidental to the increase of wall height remain unsolved.
Such increasing the noise barrier wall height permits only an improvement of sound attenuation for the added wall height. Generally speaking, the relationship between a sound attenuation by a noise barrier wall and a height of the wall is such that an incrementing in height by 1 m of the wall will result in an attenuation of about 1 dB as measured at a position 20 m or so away from a noise source.
FIG. 1 shows a conventional soundproof wall structure. As shown, the soundproof wall has a main wall 100 which stands generally vertically. The main wall 100 is provided atop thereof with a first branch wall 101 inclined towards a noise source, and also a second branch wall 102 extending obliquely in a direction opposite to the noise source. The first branch wall 101 is provided with an additional branch wall 103, while the second branch wall 102 has an additional branch wall 104. Owing to this configuration, the soundproof wall effectively acts to attenuate both a noise propagating upward from below and a one traveling downward from above, without the necessity of increasing the height thereof.
However, it has been proved that the prior-art soundproof wall as shown in FIG. 1 is limited from being further reduced in height, and that no satisfactory effect of attenuation can be expected when the wall is used against a sound of a frequency within a certain range since the noise having come into the space between the additional branch walls 103 and 104 is repeatedly reflected between the branch walls and the reflected sounds resonate with each other to cause a further noise.